The population of small mammals in different areas of the Saratov Trans-Volga region features long-term and constantly increasing numbers of Sylvaemus uralensis, and its wide distribution in a variety of habitats. The maximum density of S. uralensis is 72.0 ind. per 100 trap-days and the fraction in catches is 73.0%. A tendency is observed to an increasing numbers of Microtus arvalis.